The present invention relates to reeling machines which deploy and reel-in aerial gunnery tow targets from flying aircraft and control devices for regulating operation of such reeling machines according to towline velocity and vent door position.
In the field of automatically controlled reeling machines fitted to aircraft for reeling in and reeling out aerial gunnery targets, emphasis has been placed on regulating the rate at which the gunnery target is reeled-out or reeled-in. A type of conventional reeling machine incorporates a system for controlling turbine rotational speed. According to U.S. Pat. No. 4,770,368, an analog feed back loop is utilized in regulating the turbine speed. A tachometer provides a voltage representative of instantaneous turbine speed. The instantaneous turbine speed signal is subtracted from a reference voltage to produce an error voltage, and the error voltage is used to drive a servomotor which changes the position of the reeling machine's vent doors so as to reduce the error voltage. The vent door position, of course, controls the volume of air flowing through the turbine and thus regulates turbine rotor speed and direction.
Another general type of reeling machine provides for control according to tow cable speed rather than turbine speed. Reeling control based upon towline speed is advantageous over control according to turbine speed because, even if turbine speed is held constant, if not controlled directly, towline speed will vary as the effective tow reel diameter changes as towline is reeled-off or reeled-on the tow reel.
U.S. Pat. No. 4,496,159 shows an aerial gunnery target and reel system disclosed as including an electro-mechanical means. According to the patentees, the electro-mechanical means may include an electronic speed control means which, although it is not shown in the patent drawings is said to comprise a closed loop servo wherein actual cable speed is compared with commanded cable speed. The electro-mechanical means regulates the condition of a butterfly valve which in turn controls the volume of airflow for driving the turbine. As the target nears the reeling machine housing, cable reeling velocity is reduced by gradually closing an air inlet valve until a locking arm engages the target whereupon the valve is fully closed and a brake is applied.
Conventional reeling machines and control systems therefor are understood to make no provision for monitoring instantaneous vent door positions. If, for example, during a high speed flight, the vent doors are left open, due to component failure or even pilot error, the turbine torque may overcome the system brakes, resulting in turbine overspeed. Overspeed conditions frequently can cause extensive damage to the reeling equipment. Instantaneous vent door position therefore must be monitored to ensure that the doors are in a desired position during a reeling operation and to permit verification that the vent doors are closed and locked at all times other than during a reeling operation. In addition to component failure or power failures which may result in improperly leaving open the vent doors, conventional systems often are not equipped to adequately respond to sensor failures or counting circuitry failures. Loss of sensor data from, for example, the tachometer in a conventional machine could result in failure of its controller to slow the rotational speed of the tow reel. Thus, during a reel in operation, the target may impact the tow reel at full speed thereby causing extensive damage to both the tow reel and the target. These events are increasingly dangerous if the pilot has no indication in the cockpit that the system has failed, until a catastrophic failure occurs.
Still further, conventional systems utilizing feedback loop arrangements are often adversely affected by electromagnetic noise. Electromagnetic noise may appear as a signal causing the device mechanism for the vent doors to improperly adjust the position of the doors in response to the noise signal.